The invention generally relates to devices and methods for handover procedures in cellular networks.
An example of handover (handoff in American English) consists in switching a cellular communication from one cell to another cell during a communication, when the other cell is expected to provide a better service. Some difficulties in this process lie in particular in the determination of the criteria for deciding to trigger a handover, and then in the choice of the next cell to which to connect. For example, if all cellular devices in a given area connect to a single cell offering the best signal and none of the cellular devices connect to the neighboring cells, while the neighboring cells offer a sufficient (albeit lower) signal strength, the best cell is likely to be overloaded and to deliver a poor service, while other cells are available but unused.
A handover can be triggered by a cellular device on the basis of triggers defined by the network. The major triggers are the hysteresis, or “HO hysteresis” (for handover hysteresis), and the “Time To Trigger” (also known as TTT). A handover can be triggered when for example the RSRP value from an adjacent cell is higher than the one from the serving cell by a number of decibels greater than or equal to the parameter “HO hysteresis”. This condition has to be satisfied for a duration greater than or equal to TTT.
The cellular device may be configured to send a report to the network when some events occur, for example when the serving cell signal drops below a given threshold. The goal is to trigger an action in the network as soon as the measurement report sent by the cellular device and associated with the event is received by the network. Then the network can select a cell and instruct the cellular device to handover to such selected cell.
This handover mechanism in LTE network is explained in 3GPP TS 36.331, in particular in section 5.5.
Handover should be distinguished from cell reselection. Cell reselection in LTE network is specified in 3GPP TS 36.304. Cell reselection takes place in idle mode, is fully managed by a cellular device, and takes place while the cellular device is not communicating in the user plane. In contrast, handover takes place during a communication, and is based on information supplied by a network to a cellular device. In a handover, a network configures a cellular device to carry out measurements and communicate them to the network, and based on such measurements, the network guides the cellular device for the handover.
In addition there is no “Time To Trigger” in a reselection. Time To Trigger is used to send an event in a network. In a reselection, there is a time duration parameter called Treselection, but after Treselection has elapsed, the cellular device switches directly to the next cell without sending any information to the network (no event is triggered, and no measurement report is sent upon event triggering).
Cell reselection nonetheless comprises mechanisms which understanding can be useful in order to appreciate the invention. In particular, EP2077690A2 deals with elaborate cell reselection procedures, and will be discussed below. The approaches described below are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
According to EP2077690A2, priorities can be used for the purpose of load balancing. For example, a high priority can be set to a frequency or RAT having a low load such that a UE (user equipment, i.e. a cellular device according to LTE) preferentially selects a cell having a low load and receives a service, thereby achieving load balancing between the cells. The load balancing is achieved in the unit of cells, tracking areas, registration areas each including a plurality of tracking areas, or PLMNs, according to the range of the area to which the priority of the frequency or RAT is applied.
EP2077690A2 discloses a number of possibilities for choosing the next cell during cell reselection, two of which are noteworthy and are explained below.
A first possibility according to EP2077690A2 is illustrated on FIG. 1, which shows a method of reselecting a cell according to priority by a UE in idle mode. The priority is defined per frequency or RAT. If the priorities are defined with respect to different frequencies or RATs, the UE selects a cell corresponding to a frequency or RAT having a high priority once the cell satisfies a minimum signal characteristic value (minimum value) or a certain signal characteristic value (threshold value) defined in the system. Exceptionally, the UE may select a cell having a highest signal characteristic value without considering the priority at the time of initial cell selection. If a UE selects a cell having a highest priority and receives the service (S820) and the signal characteristic value of the cell (serving cell) which provides the service is equal to or greater than a certain value (e.g., Snonintrasearch), the measurement of a cell having a low priority may not be performed and thus power consumption can be efficiently reduced (S840 and S842). In contrast, if the signal characteristic value of the serving cell is reduced to a certain value or less although the priority of the serving cell is highest, the UE may select another cell having a low priority and that satisfies the certain signal characteristic value by the measurement process (S844 and S846). If a plural number of cells satisfy the certain value, a cell satisfying a certain criterion (e.g., a cell having a highest priority, a cell having a highest signal characteristic value or the like) may be selected from the plural number of cells (S846).
In contrast, if the UE does not select the cell using the frequency and/or RAT having the highest priority so as to receive the service (that is, the cell having a low priority is selected) (S820), although the signal characteristic value of the serving cell is equal to or greater than the certain value (e.g., Snonintrasearch), the UE periodically searches for the cell using the frequency and/or RAT having a high priority (S830 and S832). The period for searching for another cell may be jointly decided between the base station and the UE, or may be decided by any one of the base station or the UE and notified to the other node.
The cell reselection process according to a priority can take place as follows. First, when the UE reselects another cell having the same priority as the serving cell, the UE reselects a cell having a highest signal characteristic value by a ranking process of comparing the intensity and quality of the signal. The equation which is used for the ranking process between the cells having the same priority is as follows.Rs=Qmeas,s+Qhysts Rn=Qmeas,n−Qoffset
where Qmeas,s denotes a reference symbol received power (RSRP) value measured by the UE with respect to the serving cell, Qmeas,n denotes a RSRP value measured by the UE with respect to the neighbor cells, Qhysts denotes a hysteresis value for weighting the serving cell, and Qoffset denotes at least one of a bias value between cells and a bias value between different frequencies.
In the ranking process, if the cell having the highest signal characteristic value satisfies a criterion Rn>Rs for a certain time TreselectionEUTRAN, the UE selects a cell corresponding to Rn. That is, a cell having a best signal characteristic is reselected from the cells having the signal characteristic better than that of the serving cell. In the conventional WCDMA, the cell having the highest signal characteristic value is reselected by the above-described ranking process without using the priority information of the frequency or RAT.
A second possibility according to EP2077690A2 is illustrated on FIG. 2 in which a UE receives a service from a serving cell using a frequency 3. The UE performs a cell search in order to perform cell reselection. The cell reselection is performed while the signal characteristic value of the serving cell is reduced to a value of Threshserving or less (e.g., due to the movement of the UE). Assuming a cell using a frequency 1 and a cell using a frequency 2 exist in the boundary of the serving cell, the UE starts the signal measurement of the two cells. That is, the two cells become candidate cells for cell reselection. In this example, the priority of the frequency 1 is lower than that of the frequency 2. The priority of the frequency of the serving cell may be equal to that of any one of the frequency 1 or the frequency 2, or may be between the priorities of the frequency 1 and the frequency 2.
Both the signal characteristic values of the cell using the frequency 1 and the cell using the frequency 2 are equal to or greater than a certain threshold value Thresh_H/L when the UE starts the signal measurement with respect to the candidate cells. Accordingly, the UE operates a first timer with respect to the cell of the frequency 1 and monitors a time duration for which the signal characteristic value satisfies Thresh_L. In addition, the UE operates a second timer with respect to the cell of the frequency 2 and monitors a time duration for which the signal characteristic value satisfies Thresh_H.
Because the priority of the frequency 1 is lower than that of the frequency 2, the UE sets different expiration times for the first timer and the second timer (i.e., first timer: TreselectionRAT_low, second timer: TreselectionRAT_high). Specifically, the UE sets the expiration time of the first timer for the cell of the low priority to be longer than that of the second timer. If more candidate cells which are selectable by the UE are present, the UE may set the timers of these cells individually or per group in consideration of the priorities of the candidate cells.
If the timers are individually set, the UE sets the expiration time of the timer for each candidate cell according to the priority. Thus, timers become longer for cells having a lower priority.
If the timers are set per group, the UE divides the candidate cells into groups based on the priority of the candidate cell relative to the priority of the serving cell. That is, the candidate cells are divided into a) cells having a priority higher than that of the serving cell, b) cells having a priority equal to that of the serving cell, and c) cells having a priority lower than that of the serving cell. Based on this division, the expiration times of the timers of the candidate cells of the same relative priority may be set to be equal.
Because the first timer and the second timer may be simultaneously started but the expiration time of the second timer is shorter than that of the first timer, the second timer is first expired. Because the signal characteristic value of the cell using the frequency 2 is satisfied until the second timer expires, the UE stops the first timer and reselects the cell of the frequency 2. The cell of the high priority may be readily reselected by varying the length of the restriction time duration applied to the cell of the high priority. That is, it is sufficient that the restriction time duration of the cell of the high priority is relatively shorter than the cell of the low priority. And, an absolute length of each restriction time duration is not set and may be suitably determined in consideration of the wireless environment and the mobility of the UE.
To maintain backward compatibility with the conventional systems, only the time duration applied to the cell of the high priority may be set to be shorter than the conventional restriction time duration. Alternatively, only the restriction time duration applied to the cell of the low priority may be set to be longer than the conventional restriction time duration.